The multinucleated fibres are under the control of the somatic nervous system and elicit movement by forces exerted on the skeleton similar to levers and pulleys.

This fibre responds to stimulation and which it responds, it contracts and then relaxes. The muscle fibre is composed of smaller unit called myofibrils (contractile unit). Each myofibril is formed of sarcomere, which lies between the two Z-lines (membrane of Krause) of the muscle. The myofibril is composed of contractile proteins filament called actin and myosin.

The thin actin protein is also termed as light band or isotropic band or I-band whereas thick myosin protein is called dark band or anisotropic band or A-band. The thick filaments lie parallel to one another and thin filament is present in an orderly array between the thick filaments.

In the centre of the I-band, there is a band of dense amorphous material known as Z-line. In the middle of the A-band is another zone which takes a light stain known as the ‘H’ band or ‘H’ zone.

Sliding filament theory of muscle contraction:

It was proposed by A.F. Huxley and H.E. Huxley in 1954. It is also called rachet power stroke mechanism which explains the physical events involved in muscle contraction. The smallest unit of muscle contraction is a sarcomere.

When a muscle fibre is stimulated, the heads of the myosin filaments form cross bridges with actin filaments and repeatedly swivel on them. [The proteins, troponin and tropomyosin, which are closely associated with actin, are also important in regulating the attachment of actin to the cross bridges.]

This pulls actin filaments inwards over the myosin filaments. Z-lines come closer, the length of A-bands remains constant, whereas I-bands become shorter and finally disappear when the fibre is fully contracted. The H-zones also become narrow.

The energy for the muscle contraction is provided by the muscle fibre itself in the form of ATP, when the stimulus is over, the cross bridges between actin and myosin filaments are broken and the actin filaments slide back, resulting in the relaxation of the muscle fibre.